Minor edits to Description section.

Add Design Philosophy as a separate subsection.  Clarify its last sentence.
Give Automatic Mode Selection a uniquely-named anchor.
Always call the program the "test program", instead of the "initial program",
 the "LLVM program", the "test case", the "resultant module", etc.
Try to explain the assumptions a little more, instead of just describing the
 process.

llvm-svn: 9255